In recent years, the storage of hydrogen as a potential fuel or reactant, has become of increasing interest and numerous systems have been described whereby hydrogen can be stored as an interstitial hydride or stoichiometric compound of an appropriate metal, to be released as required, the storage systems being reversible.
One of these systems utilizes magnesium (Mg) which can form the hydride (MgH.sub.2) from which hydrogen can be driven in gaseous form. A storage system based upon the reversible reaction H.sub.2 +Mg=MgH.sub.2 is thus capable of storing hydrogen from a gaseous state upon contact of the hydrogen with the metal and of releasing hydrogen in a gaseous form at a subsequent time and, if desired, at a different place.
While a number of other materials have also been proposed for the storage of hydrogen in the form of respective hydrides, magnesium has been found to be of interest because of its relatively low cost and light weight which allows for a theoretical capacity of 7.6% by weight of hydrogen (based upon the weight of metal) to be stored and regenerated.
The storage of hydrogen in the form of magnesium hydride is described, for example, in French Pat. No. 1,529,371 and British Pat. No. 1,171,364.
The use of the Mg/MgH.sub.2 system for the reversible storage of hydrogen on an industrial scale, however, poses several practical problems.
For example, the magnesium should be in the form of a powder so as to obtain the maximum specific surface area for hydrogen absorption and hence for the conversion of the Mg to MgH.sub.2 under acceptable conditions.
Magnesium powder produced by thermal decomposition has a conversion ratio of MgH.sub.2 /Mg greater than 0.9. The hydride can be produced initially by permitting the magnesium to absorb hydrogen at a temperature of 327.degree. C. at a pressure of 2.3 bar for a period of 6 hours. However the cost of the MgH.sub.2 prepared by indirect techniques is too high to permit economical use of the magnesium powder thus obtained for the storage of hydrogen in the form of a hydride by most economic criteria.
Furthermore, efforts to use magnesium turnings, instead of magnesium powder, for the storage of hydrogen have required conditions so extreme as to render the system impractical notwithstanding the lower cost of the starting material. For example, fine magnesium turnings must be maintained for several days at a temperature of 400.degree. to 450.degree. C. at a pressure of 70 bar in a hydrogen atmosphere for a molar conversion ratio MgH.sub.2 /Mg of 0.9. These absorption conditions cannot be readily realized without special equipment and make the storage of hydrogen on an industrial scale impractical wherever these conditions must be observed.
Finally it is a practical necessity to provide conditions which can be generated conveniently and economically for absorption and desorption of hydrogen and yet provide a conversion ratio of MgH.sub.2 /Mg which is as high as possible to obtain best utilization of magnesium. In other words this molar ratio should be as high as possible, the duration required for absorption and desorption should be as low as possible and both the absorption temperature and absorption pressure should be kept as low as possible.
It has been proposed to reduce the absorption temperature, the absorption pressure and the duration required for complete hydrogenation of magnesium by providing the magnesium in the form of an alloy with copper or nickel, namely, intermetallic compounds such as Mg.sub.2 Cu and Mg.sub.2 Ni. The advantages of these materials is that they allow practically complete transformation of magnesium to MgH.sub.2 at a temperature of 200.degree. C. and 300.degree. C. with a hydrogen pressure of 15 bar.
The state of the art relating to the storage of hydrogen in magnesium alloys is illustrated by the following works and publications:
D. L. Douglas: The Storage and Release of Hydrogen From Magnesium Alloy Hydrides for Vehicular Applications, International Symposium on Hydrides for Energy Storage, Norway, August 1977;
"Preparation of Magnesium Hydride", Russian Journal of Inorganic Chemistry, pp. 389-395, April 1961;
"The Reaction of Hydrogen with Alloys of Magnesium and Copper", Inorganic Chemistry, Vol. 6, No. 12, December 1967; and
"The Reaction of Hydrogen with Alloys of Magnesium and Nickel and the Formation of Mg.sub.2 NiH.sub.4 ", Inorganic Chemistry, Vol. 7, No. 11, November 1967.
A disadvantage of magnesium alloys for the purposes described is that the magnesium alloys are comparatively costly since they must be prepared by smelting the elements, casting the resulting melt, comminuting the cast body and milling the comminuted product to a fine powder capable of absorbing hydrogen rapidly.
It has also been proposed to provide catalysts for an increase in the reaction rate of hydrogen with the magnesium powder. Such catalysts can be organic compounds, generally organohalides or metals or alloys, especially titanium, vanadium, LaNi.sub.5 and TiFe, which are known to react readily with hydrogen to form respective hydrides as is described in French application No. 75 28 647.
Such catalysts are additives whose use may be incompatible with industrial exploitation of magnesium-based hydrogen storage systems for industrial purposes and, naturally, increase the cost of the system and may introduce factors which affect the reliability of magnesium as a hydrogen storage metal.